Li Yifei thought Meng Xiaofei was a Meng Family Member for a reason. Meng Xiaofei had mentioned before that she was from a major family and left early because she disliked the atmosphere at home. The only prominent family named Meng seemed to be the one in Capital City.
After Li Yifei killed Meng Xiaojun and returned to Mile City, Meng Xiaofei also happened to return to Mile City and mentioned that she attended a friend's funeral. It was clear that she must have attended Meng Xiaojun's funeral. Although she didn't like the Meng Family, Meng Xiaojun was still her relative. Even if they weren't siblings, they could have been cousins.